Search space for EPDCCH control information in an OFDM-based mobile communication system

ABSTRACT

The present invention relates to a method for receiving control information within a subframe of a multi-carrier communication system supporting carrier aggregation, the method comprising the following steps performed at a receiving node: performing a blind detection for the control information within a search space by means of a first search pattern, wherein the first search pattern is one of a plurality of search patterns, each of the plurality of search patterns comprising a plurality of candidates distributed on any of a plurality of aggregation levels, and wherein the plurality of search patterns further comprises a second search pattern whose candidates are non-overlapping the candidates of the first search pattern on the same aggregation levels.

The present invention relates to methods and apparatuses forconfiguration of search space and to search space channel structure forsignaling of the control information.

Third generation (3G) mobile systems, such as, for instance, universalmobile telecommunication systems (UMTS) standardized within the thirdgeneration partnership project (3GPP) have been based on wideband codedivision multiple access (WCDMA) radio access technology. Today, 3Gsystems are being deployed on a broad scale all around the world. Afterenhancing this technology by introducing high-speed downlink packetaccess (HSDPA) and an enhanced uplink, also referred to as high-speeduplink packet access (HSUPA), the next major step in evolution of theUMTS standard has brought the combination of orthogonal frequencydivision multiplexing (OFDM) for the downlink and single carrierfrequency division multiplexing access (SC-FDMA) for the uplink. Thissystem has been named long term evolution (LTE) since it has beenintended to cope with future technology evolutions.

The LTE system represents efficient packet based radio access and radioaccess networks that provide full IP-based functionalities with lowlatency and low cost. The detailed system requirements are given in 3GPPTR 25.913, “Requirements for evolved UTRA (E-UTRA) and evolved UTRAN(E-UTRAN),” v8.0.0, December 2008, (available at http://www.3gpp.org/and incorporated herein by reference). The Downlink will support datamodulation schemes QPSK, 16QAM, and 64QAM and the Uplink will supportBPSK, QPSK, 8PSK and 16QAM.

LTE's network access is to be extremely flexible, using a number ofdefined channel bandwidths between 1.25 and 20 MHz, contrasted with UMTSterrestrial radio access (UTRA) fixed 5 MHz channels. Spectralefficiency is increased by up to four-fold compared with UTRA, andimprovements in architecture and signaling reduce round-trip latency.Multiple Input/Multiple Output (MIMO) antenna technology should enable10 times as many users per cell as 3GPP's original WCDMA radio accesstechnology. To suit as many frequency band allocation arrangements aspossible, both paired (frequency division duplex FDD) and unpaired (timedivision duplex TDD) band operation is supported. LTE can co-exist withearlier 3GPP radio technologies, even in adjacent channels, and callscan be handed over to and from all 3GPP's previous radio accesstechnologies.

FIG. 1 illustrates structure of a component carrier in LTE Release 8.The downlink component carrier of the 3GPP LTE Release 8 is sub-dividedin the time-frequency domain in so-called sub-frames each of which isdivided into two downlink slots 120 corresponding to a time periodT_(slot). The first downlink slot comprises a control channel regionwithin the first OFDM symbol(s). Each sub-frame consists of a givennumber of OFDM symbols in the time domain, each OFDM symbol spanningover the entire bandwidth of the component carrier.

The smallest unit of resources that can be assigned by a scheduler is aresource block 130 also called physical resource block (PRB). A PRB 130is defined as N_(symb) ^(DL) consecutive OFDM symbols in the time domainand N_(sc) ^(RB) consecutive sub-carriers in the frequency domain. Inpractice, the downlink resources are assigned in resource block pairs. Aresource block pair consists of two resource blocks. It spans N_(sc)^(RB) consecutive sub-carriers in the frequency domain and the entire2·N_(symb) ^(DL) modulation symbols of the sub-frame in the time domain.N_(symb) ^(DL) may be either 6 or 7 resulting in either 12 or 14 OFDMsymbols in total. Consequently, a physical resource block 130 consistsof N_(symb) ^(DL)×N_(sc) ^(RB) resource elements 140 corresponding toone slot in the time domain and 180 kHz in the frequency domain (furtherdetails on the downlink resource grid can be found, for example, in 3GPPTS 36.211, “Evolved universal terrestrial radio access (E-UTRA);physical channels and modulations (Release 8)”, version 8.9.0, December2009, Section 6.2, available at http://www.3gpp.org. which isincorporated herein by reference).

The number of physical resource blocks N_(RB) ^(DL) in downlink dependson the downlink transmission bandwidth configured in the cell and is atpresent defined in LTE as being from the interval of 6 to 110 PRBs.

The data are mapped onto physical resource blocks by means of pairs ofvirtual resource blocks. A pair of virtual resource blocks is mappedonto a pair of physical resource blocks. The following two types ofvirtual resource blocks are defined according to their mapping on thephysical resource blocks in LTE downlink:

-   -   Localized Virtual Resource Block (LVRB)    -   Distributed Virtual Resource Block (DVRB)

In the localized transmission mode using the localized VRBs, the eNB hasfull control which and how many resource blocks are used, and should usethis control usually to pick resource blocks that result in a largespectral efficiency. In most mobile communication systems, this resultsin adjacent physical resource blocks or multiple clusters of adjacentphysical resource blocks for the transmission to a single userequipment, because the radio channel is coherent in the frequencydomain, implying that if one physical resource block offers a largespectral efficiency, then it is very likely that an adjacent physicalresource block offers a similarly large spectral efficiency. In thedistributed transmission mode using the distributed VRBs, the physicalresource blocks carrying data for the same UE are distributed across thefrequency band in order to hit at least some physical resource blocksthat offer a sufficiently large spectral efficiency, thereby obtainingfrequency diversity.

In 3GPP LTE Release 8 there is only one component carrier in uplink anddownlink. Downlink control signaling is basically carried by thefollowing three physical channels:

-   -   Physical control format indicator channel (PCFICH) for        indicating the number of OFDM symbols used for control signaling        in a sub-frame (i.e. the size of the control channel region);    -   Physical hybrid ARQ indicator channel (PHICH) for carrying the        downlink ACK/NACK associated with uplink data transmission; and    -   Physical downlink control channel (PDCCH) for carrying downlink        scheduling assignments and uplink scheduling assignments.

The PCFICH is sent from a known position within the control signalingregion of a downlink sub-frame using a known pre-defined modulation andcoding scheme. The user equipment decodes the PCFICH in order to obtaininformation about a size of the control signaling region in a sub-frame,for instance, the number of OFDM symbols. If the user equipment (UE) isunable to decode the PCFICH or if it obtains an erroneous PCFICH value,it will not be able to correctly decode the L1/L2 control signaling(PDCCH) comprised in the control signaling region, which may result inlosing all resource assignments contained therein.

The PDCCH carries control information, such as, for instance, schedulinggrants for allocating resources for downlink or uplink datatransmission. A physical control channel is transmitted on anaggregation of one or several consecutive control channel elements(CCEs). Each CCE corresponds to a set of resource elements grouped toso-called resource element groups (REG). A control channel elementtypically corresponds to 9 resource element groups. A scheduling granton PDCCH is defined based on control channel elements (CCE). Resourceelement groups are used for defining the mapping of control channels toresource elements. Each REG consists of four consecutive resourceelements excluding reference signals within the same OFDM symbol. REGsexist in the first one to four OFDM symbols within one sub-frame. ThePDCCH for the user equipment is transmitted on the first of either one,two or three OFDM symbols according to PCFICH within a sub-frame.

Another logical unit used in mapping of data onto physical resources in3GPP LTE Release 8 (and later releases) is a resource block group (RBG).A resource block group is a set of consecutive (in frequency) physicalresource blocks. The concept of RBG provides a possibility of addressingparticular RBGs for the purpose of indicating a position of resourcesallocated for a receiving node (e.g. UE), in order to minimize theoverhead for such an indication, thereby decreasing the control overheadto data ratio for a transmission. The size of RBG is currently specifiedto be 1, 2, 3, or 4, depending on the system bandwidth, in particular,on N_(RB) ^(DL). Further details of RBG mapping for PDCCH in LTE Release8 may be found in 3GPP TS 36.213 “Evolved Universal terrestrial RadioAccess (E-UTRA); Physical layer procedures”, v8.8.0, September 2009,Section 7.1.6.1, freely available http://www.3gpp org/ and incorporatedherein by reference.

Physical downlink shared channel (PDSCH) is used to transport user data.PDSCH is mapped to the remaining OFDM symbols within one sub-frame afterPDCCH. The PDSCH resources allocated for one UE are in the units ofresource block for each sub-frame.

FIG. 2 shows an exemplary mapping of PDCCH and PDSCH within a sub-frame.The first two OFDM symbols form a control channel region (PDCCH region)and are used for L1/L2 control signaling. The remaining twelve OFDMsymbols form data channel region (PDSCH region) and are used for data.Within a resource block pairs of all sub-frames, cell-specific referencesignals, so-called common reference signals (CRS), are transmitted onone or several antenna ports 0 to 3. In the example of FIG. 2, the CRSare transmitted from two antenna ports: R0 and R1. Moreover, thesub-frame also includes UE-specific reference signals, so-calleddemodulation reference signals (DM-RS) used by the user equipment fordemodulating the PDSCH. The DM-RS are only transmitted within theresource blocks in which the PDSCH is allocated for a certain userequipment. In order to support multiple input/multiple output (MIMO)with DM-RS, four DM-RS layers are defined meaning that at most, MIMO offour layers is supported. In this example, in FIG. 2, DM-RS layer 1, 2,3 and 4 are corresponding to MIMO layer 1, 2, 3 and 4.

One of the key features of LTE is the possibility to transmit multicastor broadcast data from multiple cells over a synchronized singlefrequency network which is known as multimedia broadcast singlefrequency network (MBSFN) operation. In MBSFN operation, UE receives andcombines synchronized signals from multiple cells. To facilitate this,UE needs to perform a separate channel estimation based on an MBSFNreference signal. In order to avoid mixing the MBSFN reference signaland normal reference signal in the same sub-frame, certain sub-framesknown as MB SFN sub-frames are reserved from MBSFN transmission.

The structure of an MBSFN sub-frame is shown in FIG. 3 up to two of thefirst OFDM symbols are reserved for non-MB SFN transmission and theremaining OFDM symbols are used for MBSFN transmission. In the first upto two OFDM symbols, PDCCH for uplink resource assignments and PHICH canbe transmitted and the cell-specific reference signal is the same asnon-MBSFN transmission sub-frames. The particular pattern of MBSFNsub-frames in one cell is broadcasted in the system information of thecell. UEs not capable of receiving MBSFN will decode the first up to twoOFDM symbols and ignore the remaining OFDM symbols. MBSFN sub-frameconfiguration supports both 10 ms and 40 ms periodicity. However,sub-frames with number 0, 4, 5 and 9 cannot be configured as MBSFNsub-frames. FIG. 3 illustrates the format of an MBSFN subframe. ThePDCCH information sent on the L1/L2 control signaling may be separatedinto the shared control information and dedicated control information.

The frequency spectrum for IMT-advanced was decided at the World RadioCommunication Conference (WRC-07) in November 2008. However, the actualavailable frequency bandwidth may differ for each region or country. Theenhancement of LTE standardized by 3GPP is called LTE-advanced (LTE-A)and has been approved as the subject matter of Release 10. LTE-A Release10 employs carrier aggregation according to which two or more componentcarriers as defined for LTE Release 8 are aggregated in order to supportwider transmission bandwidth, for instance, transmission bandwidth up to100 MHz. More details on carrier aggregation can be found in 3GPP TS36.300 “Evolved Universal terrestrial Radio Access (E-UTRA) andUniversal terrestrial Radio Access Network (E-UTRAN); Overalldescription”, v10.2.0, December 2010, Section 5.5 (Physical layer),Section 6.4 (Layer 2) and Section 7.5 (RRC), freely available athttp://www.3gpp.org/ and incorporated herein by reference. It iscommonly assumed that the single component carrier does not exceed abandwidth of 20 MHz. A terminal may simultaneously receive and/ortransmit on one or multiple component carriers depending on itscapabilities. A UE may be configured to aggregate a different number ofcomponent carriers (CC) in the uplink and in the downlink. The number ofdownlink CCs which can be configured depends on the downlink aggregationcapability of the UE. The number of uplink CCs which can be configureddepends on the uplink aggregation capability of the UE. However, it isnot possible to configure a UE with more uplink CCs than downlink CCs.

The term “component carrier” is sometimes replaces with the term “cell”since, similar to a concept of a cell known from earlier releases of LTEand UMTS, a component carrier defines resources fortransmission/reception of data and may be added/reconfigures/removedfrom the resources utilized by the wireless nodes (e.g. UE, RN). Inparticular, a cell is a combination of downlink and optionally uplinkresources, i.e. downlink and optional uplink component carrier. InRel-8/9, there are one carrier frequency of downlink resources and onecarrier frequency of uplink resources. The carrier frequency of downlinkresources is detected by UE through cell selection procedure. Thecarrier frequency of uplink resources is informed to UE through SystemInformation Block 2. When carrier aggregation is configured, there aremore than one carrier frequency of downlink resources and possibly morethan one carrier frequency of uplink resources. Therefore, there wouldbe more than one combination of downlink and optionally uplinkresources, i.e. more than one serving cell. The primary serving cell iscalled Primary Cell (PCell). Other serving cells are called SecondaryCells (SCells).

When carrier aggregation is configured, a UE has only one Radio ResourceControl (RRC) connection with the network. Primary Cell (PCell) providesthe non-access stratum (NAS) mobility information and security input atRRC connection reestablishment or handover. Depending on UEcapabilities, Secondary Cells (SCells) can be configured to formtogether with the PCell a set of serving cells. RRC connection is theconnection between RRC layer on UE side and RRC layer on network side.Establishment, maintenance and release of an RRC connection between theUE and E-UTRAN include: allocation of temporary identifiers between UEand E-UTRAN; configuration of signaling radio bearer(s) for RRCconnection, i.e, Low priority SRB and high priority SRB. More details onRRC can be found in 3GPP TS 36.331 “Evolved Universal terrestrial RadioAccess (E-UTRA); Radio Resource Control (RRC); Protocol specification”,v10.0.0, December 2010, freely available at http://www.3gpp.org/ andincorporated herein by reference.

In the downlink, the carrier corresponding to PCell is called DownlinkPrimary Component Carrier (DL PCC) whereas in the uplink, the carriercorresponding to PCell is called Uplink Primary Component Carrier (ULPCC). The linking between DL PCC and UL PCC is indicated in the systeminformation (System Information Block 2) from the PCell. Systeminformation is common control information broadcast by each cell,including, for instance, information about the cell to the terminals.With regard to the system information reception for the PCell, theprocedure of LTE in Rel-8/9 applies. The details on system informationreception procedure for Rel-8/9 can be found in 3GPP TS 36.331 “EvolvedUniversal terrestrial Radio Access (E-UTRA); Radio Resource Control(RRC); Protocol specification”, v9.5.0, December 2010, Section 5.2,freely available at http://www.3gpp.org/ and incorporated herein byreference. In the downlink, the carrier corresponding to an SCell is aDownlink Secondary Component Carrier (DL SCC) while in the uplink it isan Uplink Secondary Component Carrier (UL SCC). The linking between DLSCC and UL SCC is indicated in the system information (SystemInformation Block 2) of the SCell. All required system information ofthe SCell is transmitted to UE through dedicated RRC signaling whenadding an SCell. Hence, there is no need for the UE to acquire systeminformation directly from SCells. The system information of an SCellremains valid as long as the SCell is configured. Changes in systeminformation of an SCell are handled through the removal and addition ofthe SCell. Removal and/or addition of an SCell can be performed using anRRC procedure.

Both downlink grant and uplink grant are received on DL CC. Therefore,in order to know the uplink grant received on one DL CC corresponds tothe uplink transmission of which UL CC, the linking between DL CC and ULCC would be necessary.

A linking between UL CC and DL CC allows identifying the serving cellfor which the grant applies:

-   -   downlink assignment received in PCell corresponds to downlink        transmission in the PCell,    -   uplink grant received in PCell corresponds to uplink        transmission in the PCell,    -   downlink assignment received in SCell_(N) corresponds to        downlink transmission in the SCell_(N),    -   uplink grant received in SCell_(N) corresponds to uplink        transmission in the SCell_(N). If SCell_(N) is not configured        for uplink usage by the UE, the grant is ignored by the UE.

3GPP TS 36.212 v10.0.0, also describes in Section 5.3.3.1 thepossibility of cross-carrier scheduling, using a Carrier IndicationField (CIF).

UE may be scheduled over multiple serving cells simultaneously. Across-carrier scheduling with a CIF allows the PDCCH of a serving cellto schedule resources in another serving cell(s), however, with thefollowing restrictions:

-   -   cross-carrier scheduling does not apply to PCell, which means        that PCell is always scheduled via its own PDCCH,    -   when the PDCCH of a secondary cell (SCell) is configured,        cross-carrier scheduling does not apply to this SCell, which        means that the SCell is always scheduled via its own PDCCH, and    -   when the PDCCH of an SCell is not configured, cross-carrier        scheduling applies and such SCell is always scheduled via PDCCH        of another serving cell.

Therefore, if there is no CIF, the linking between DL CC and UL CCidentifies the UL CC for uplink transmission; if there is CIF, the CIFvalue identifies the UL CC for uplink transmission.

The set of PDCCH candidates to monitor, where monitoring impliesattempting to decode each of the PDCCHs, are defined in terms of searchspaces. A UE not configured with a Carrier Indicator Field (CIF) shallmonitor one UE-specific search space at each of the aggregation levels1,2,4,8 on each activated serving cell. A UE configured with a CarrierIndicator Field (CIF) shall monitor one or more UE-specific searchspaces at each of the aggregation levels 1,2,4,8 on one or moreactivated serving cells. If a UE is configured with a CIF, the UEspecific search space is determined by the component carrier, whichmeans that the indices of CCEs corresponding to PDCCH candidates of thesearch space are determined by the Carrier Indicator Field (CIF) value.The carrier indicator field specifies an index of a component carrier.

If a UE is configured to monitor PDCCH candidates in a given servingcell with a given DCI format size with CIF, the UE shall assume that aPDCCH candidate with the given DCI format size may be transmitted in thegiven serving cell in any UE specific search space corresponding to anyof the possible values of CIF for the given DCI format size. It meansthat if one given DCI format size can have more than one CIF value, UEshall monitor the PDCCH candidates in any UE specific search spacescorresponding to any possible CIF value with that given DCI format.

Further details on configurations of search spaces with and without CIFas defined in LTE-A for PDCCH can be found in 3GPP TS 36.213 “EvolvedUniversal terrestrial Radio Access (E-UTRA); Physical Layer procedures”,v10.0.0, December 2010, Section 9.1.1, freely available athttp://www.3gpp.org/ and incorporated herein by reference.

Another key feature of the LTE-A is providing relaying functionality bymeans of introducing relay nodes to the UTRAN architecture of 3GPPLTE-A. Relaying is considered for LTE-A as a tool for improving thecoverage of high data rates, group mobility, temporary networkdeployment, the cell edge throughput and/or to provide coverage in newareas.

A relay node is wirelessly connected to radio access network via a donorcell. Depending on the relaying strategy, a relay node may be part ofthe donor cell or, alternatively, may control the cells on its own. Incase the relay node is a part of the donor cell, the relay node does nothave a cell identity on its own, however, may still have a relay ID. Inthe case the relay node controls cells on its own, it controls one orseveral cells and a unique physical layer cell identity is provided ineach of the cells controlled by the relay. At least, “type 1” relaynodes will be a part of 3GPP LTE-A. A “type 1” relay node is a relayingnode characterized by the following:

-   -   The relay node controls cells each of which appears to a user        equipment as a separate cell distinct from the donor cell.    -   The cells should have its own physical cell ID as defined in LTE        Release 8 and the relay node shall transmit its own        synchronization channels, reference symbols etc.    -   Regarding the single cell operation, the UE should receive        scheduling information and HARQ feedback directly from the relay        node and send its controlled information (acknowledgments,        channel quality indications, scheduling requests) to the relay        node.    -   The relay node should appear as a 3GPP LTE compliant eNodeB to        3GPP LTE compliant user equipment in order to support the        backward compatibility.    -   The relay node should appear differently to the 3GPP LTE eNodeB        in order to allow for further performance enhancements to the        3GPP LTE-A compliant user equipments.

FIG. 4 illustrates an example 3GPP LTE-A network structure using relaynodes. A donor eNodeB (d-eNB) 410 directly serves a user equipment UE1415 and a relay node (RN) 420 which further serves UE2 425. The linkbetween donor eNodeB 410 and the relay node 420 is typically referred toas relay backhaul uplink/downlink. The link between the relay node 420and user equipment 425 attached to the relay node (also denoted r-UEs)is called (relay) access link.

The donor eNodeB transmits L1/L2 control and data to the micro-userequipment UE1 415 and also to a relay node 420 which further transmitsthe L1/L2 control and data to the relay-user equipment UE2 425. Therelay node may operate in a so-called time multiplexing mode, in whichtransmission and reception operation cannot be performed at the sametime. In particular, if the link from eNodeB 410 to relay node 420operates in the same frequency spectrum as the link from relay node 420to UE2 425, due to the relay transmitter causing interference to its ownreceiver, simultaneous eNodeB-to-relay node and relay node-to-UEtransmissions on the same frequency resources may not be possible unlesssufficient isolation of the outgoing and incoming signals is provided.Thus, when relay node 420 transmits to donor eNodeB 410, it cannot, atthe same time, receive from UEs 425 attached to the relay node.Similarly, when a relay node 420 receives data from donor eNodeB, itcannot transmit data to UEs 425 attached to the relay node. Thus, thereis a sub-frame partitioning between relay backhaul link and relay accesslink.

Regarding the support of relay nodes, in 3GPP it has currently beenagreed that:

-   -   Relay backhaul downlink sub-frames during which eNodeB to relay        downlink backhaul transmission is configured, are        semi-statically assigned.    -   Relay backhaul uplink sub-frames during which relay-to-eNodeB        uplink backhaul transmission is configured are semi-statically        assigned or implicitly derived by HARQ timing from relay        backhaul downlink sub-frames.    -   In relay backhaul downlink sub-frames, a relay node will        transmit to donor eNodeB and consequently r-UEs are not supposed        to expect receiving any data from the relay node. In order to        support backward compatibility for UEs that are not aware of        their attachment to a relay node (such as Release 8 UEs for        which a relay node appears to be a standard eNodeB), the relay        node configures backhaul downlink sub-frames as MBSFN        sub-frames.

In the following, a network configuration as shown in FIG. 4 is assumedfor exemplary purposes. The donor eNodeB transmits L1/L2 control anddata to the macro-user equipment (UE1) and 410 also to the relay (relaynode) 420, and the relay node 420 transmits L1/L2 control and data tothe relay-user equipment (UE2) 425. Further assuming that the relay nodeoperates in a time-duplexing mode, i.e. transmission and receptionoperation are not performed at the same time. Whenever the relay node isin “transmit” mode, UE2 needs to receive the L1/L2 control channel andphysical downlink shared channel (PDSCH), while when the relay node isin “receive” mode, i.e. it is receiving L1/L2 control channel and PDSCHfrom the Node B, it cannot transmit to UE2 and therefore UE2 cannotreceive any information from the relay node in such a sub-frame. In thecase that the UE2 is not aware that it is attached to a relay node (forinstance, a Release-8 UE), the relay node 420 has to behave as a normal(e-)NodeB. As will be understood by those skilled in the art, in acommunication system without relay node any user equipment can alwaysassume that at least the L1/L2 control signal is present in everysub-frame. In order to support such a user equipment in operationbeneath a relay node, the relay node should therefore pretend such anexpected behavior in all sub-frames.

As shown in FIGS. 2 and 3, each downlink sub-frame consists of twoparts, control channel region and data region. FIG. 5 illustrates anexample of configuring MBSFN frames on relay access link in situation,in which relay backhaul transmission takes place. Each subframecomprises a control data portion 510, 520 and a data portion 530, 540.The first OFDM symbols 720 in an MBSFN subframe are used by the relaynode 420 to transmit control symbols to the r-UEs 425. In the remainingpart of the sub-frame, the relay node may receive data 540 from thedonor eNodeB 410. Thus, there cannot be any transmission from the relaynode 420 to the r-UE 425 in the same sub-frame. The r-UE receives thefirst up to two OFDM control symbols and ignores the remaining part ofthe sub-frame. Non-MBSFN sub-frames are transmitted from the relay node420 to the r-UE 525 and the control symbols 510 as well as the datasymbols 530 are processed by the r-UE 425. An MBSFN sub-frame can beconfigured for every 10 ms on every 40 ms. Thus, the relay backhauldownlink sub-frames also support both 10 ms and 40 ms configurations.Similarly to the MBSFN sub-frame configuration, the relay backhauldownlink sub-frames cannot be configured at sub-frames with #0, #4, #5and #9. Those subframes that are not allowed to be configured asbackhaul DL subframes are called “illegal DL subframes”. Thus, relay DLbackhaul subframes can be normal or MBSFN subframe on d-eNB side.Currently it is agreed that relay backhaul DL subframes, during whicheNB 410 to relay node 420 downlink backhaul transmission may occur, aresemi-statically assigned. Relay backhaul UL subframes, during whichrelay node 420 to eNB 410 uplink backhaul transmission may occur, aresemi-statically assigned or implicitly derived by HARQ timing from relaybackhaul DL subframes.

Since MBSFN sub-frames are configured at relay nodes as downlinkbackhaul downlink sub-frames, the relay node cannot receive PDCCH fromthe donor eNodeB. Therefore, a new physical control channel (R-PDCCH) isused to dynamically or “semi-persistently” assign resources within thesemi-statically assigned sub-frames for the downlink and uplink backhauldata. The downlink backhaul data is transmitted on a new physical datachannel (R-PDSCH) and the uplink backhaul data is transmitted on a newphysical data channel (R-PUSCH). The R-PDCCH(s) for the relay nodeis/are mapped to an R-PDCCH region within the PDSCH region of thesub-frame. The relay node expects to receive R-PDCCH within the regionof the sub-frame. In time domain, the R-PDCCH region spans theconfigured downlink backhaul sub-frames. In frequency domain, theR-PDCCH region exists on certain resource blocks preconfigured for therelay node by higher layer signaling. Regarding the design and use of anR-PDCCH region within a sub-frame, the following characteristics havebeen agreed in standardization:

-   -   R-PDCCH is assigned PRBs for transmission semi-statically.        Moreover, the set of resources to be currently used for R-PDCCH        transmission within the above semi-statically assigned PRBs may        vary dynamically, between sub-frames.    -   The dynamically configurable resources may cover the full set of        OFDM symbols available for the backhaul link or may be        constrained to their sub-set.    -   The resources that are not used for R-PDCCH within the        semi-statically assigned PRBs may be used to carry R-PDSCH or        PDSCH.    -   In case of MB SFN sub-frames, the relay node transmits control        signals to the r-UEs. Then, it can become necessary to switch        transmitting to receiving mode so that the relay node may        receive data transmitted by the donor eNodeB within the same        sub-frame. In addition to this gap, the propagation delay for        the signal between the donor eNodeB and the relay node has to be        taken into account. Thus, the R-PDCCH is first transmitted        starting from an OFDM symbol which, within the sub-frame, is        late enough in order for a relay node to receive it.    -   The mapping of R-PDCCH on the physical resources may be        performed either in a frequency distributed manner or in a        frequency localized manner.    -   The interleaving of R-PDCCH within the limited number of PRBs        can achieve diversity gain and, at the same time, limit the        number of PRBs wasted.    -   In non-MBSFN sub-frames, Release 10 DM-RS is used when DM-RS are        configured by ENodeB. Otherwise, Release 8 CRS are used. In        MBSFN sub-frames, Release 10 DM-RS are used.    -   R-PDCCH can be used for assigning downlink grant or uplink grant        for the backhaul link. The boundary of downlink grant search        space and uplink grant search space is a slot boundary of the        sub-frame. In particular, the downlink grant is only transmitted        in the first slot and the uplink grant is only transmitted in        the second slot of the sub-frame.    -   No interleaving is applied when demodulating with DM-RS. When        demodulating with CRS, both REG level interleaving and no        interleaving are supported.

Relay backhaul R-PDCCH search space is a region where relay node 420expects to receive R-PDCCHs. In time domain, it exists on the configuredDL backhaul subframes. In frequency domain, it exists on certainresource blocks that are configured for relay node 420 by higher layersignaling. R-PDCCH can be used for assigning DL grant or UL grant forthe backhaul link.

According to agreements reached in RANI about the characteristics of therelay backhaul R-PDCCH in no cross-interleaving case, a UE-specificsearch space has following properties:

-   -   Each R-PDCCH candidate contains continuous VRBs,    -   The set of VRBs is configured by higher layers using resource        allocation types 0, 1, or 2,    -   The same set of VRBs is configured for a potential R-PDCCH in        the first and in the second slot,    -   DL grant is only received in 1st slot and UL grant is only        received in 2nd slot, and    -   The number of candidates for the respective aggregation level        {1,2,4,8} is {6,6,2,2}.

R-PDCCH without cross-interleaving means that, an R-PDCCH can betransmitted on one or several PRBs without being cross-interleaved withother R-PDCCHs in a given PRB. In the frequency domain, the set of VRBsis configured by higher layer using resource allocation types 0, 1, or 2according to Section 7.1.6 of 3GPP TS 36.213 “Evolved Universalterrestrial Radio Access (E-UTRA); Physical layer procedures”, v8.8.0,September 2009, freely available at http://www.3gpp.org/ andincorporated herein by reference. If the set of VRBs is configured byresource allocation type 2 with distributed

VRB to PRB mapping, the provisions in Section 6.2.3.2 of 3GPP TS 36.211for even slot numbers are always applied. The details can be found in3GPP TS 36.211, “Evolved universal terrestrial radio access (E-UTRA);physical channels and modulation (Release 8)”, version 8.9.0, December2009, Section 6.2, available at http://www.3gpp.org, which isincorporated herein by reference.

The UE usually monitors a set of PDCCH candidates on the serving cellfor control information in every non-DRX subframe, where monitoringimplies attempting to decode each of the PDCCHs in the set according toall the monitored DCI formats. The set of PDCCH candidates to monitorare defined in terms of search spaces.

UE monitors two types of search space: UE specific search space andcommon search space. Both UE specific search space and common searchspace have different aggregation levels.

In UE specific search space, there are {6,6,2,2} number of PDCCHcandidates at aggregation level {1,2,4,8} and the PDCCH candidates ofeach aggregation level are consecutive in CCEs. The starting CCE indexof the first PDCCH candidate in aggregation level L is decided byY_(k)×L. k is the subframe number and Y_(k) is decided by k and UE ID.Therefore, the positions of CCEs in UE specific search space are decidedby UE ID to reduce the overlap of PDCCH UE specific search space fromdifferent UEs and are randomized from subframe to subframe to randomizedthe interference from PDCCH in neighboring cells.

In common search space, there are {4,2} number of PDCCH candidates ataggregation level {4,8}. The first PDCCH candidate in aggregation levelL starts from CCE index 0. Therefore, all the UEs monitor the samecommon search space.

PDCCH for system information is transmitted in common search space, sothat all the UEs can receive system information by monitoring commonsearch space.

The same also applies in ePDCCH. in ePDCCH, In particular, it iscustomary to use antenna ports 7-10 for ePDCCH demodulation. Bothlocalized and distributed transmission of ePDCCH are supported.

A full flexible configuration of the search space and the antenna ports(APs) can be used for ePDCCH. However, such approach results in a largesignaling overhead while the benefits are minimal.

In view of the above, the aim of the present invention is to provide anefficient scheme for configuring a search space in which controlinformation can be signaled to a receiver. In particular, it is anobject of the invention to provide a configuration of the search spacesuch that flexibility is maintained, while the signaling overhead isminimized.

This is achieved by the teaching of the independent claims.

Advantageous embodiments of the invention are subject to the dependentclaims.

In particular, the present invention can relate to a method forreceiving control information within a subframe of a multi-carriercommunication system supporting carrier aggregation, the methodcomprising the following steps performed at a receiving node: performinga blind detection for the control information within a search space bymeans of a first search pattern, wherein the first search pattern is oneof a plurality of search patterns, each of the plurality of searchpatterns comprising a plurality of candidates distributed on any of aplurality of aggregation levels, and wherein the plurality of searchpatterns further comprises a second search pattern whose candidates arenon-overlapping the candidates of the first search pattern on the sameaggregation levels.

Additionally, the invention can relate to a method for transmittingcontrol information for at least one receiving node within a subframe ofa multi-carrier communication system supporting carrier aggregation, themethod comprising the following steps performed at the transmittingnode: mapping control information for the receiving node onto a searchspace by means of a first search pattern, wherein the first searchpattern is one of a plurality of search patterns, each of the pluralityof search patterns comprising a plurality of candidates distributed onany of a plurality of aggregation levels, and transmitting the subframeto the receiving node, wherein the plurality of search patterns furthercomprises a second search pattern whose candidates are non-overlappingthe candidates of the first search pattern on the same aggregationlevels.

In further advantageous embodiments, the first search pattern cancomprise the same plurality of aggregation levels as the second searchpattern and wherein the number of candidates, on any given aggregationlevel, of the first search pattern can correspond to the number ofcandidates on the same aggregation level of the second search pattern.

In further advantageous embodiments, the plurality of search patternscan further comprise a third search pattern whose candidates arenon-overlapping candidates of the first search pattern on the sameaggregation levels.

In further advantageous embodiments, the first search pattern and thethird search pattern can have both at least one common aggregation leveland wherein the number of candidates of the first search pattern, on thecommon aggregation level, can correspond to the number of candidates ofthe third search pattern, on the common aggregation level.

In further advantageous embodiments, the plurality of search patternscan further comprise a fourth search pattern which comprises candidatesonly within its largest aggregation level.

In further advantageous embodiments, any of the plurality of the searchpatterns can comprise the candidates which is non-overlapping with eachother on the same aggregation level.

In further advantageous embodiments, any of the plurality of the searchpatterns can comprise the candidates which are non-overlapping with eachother on any of the plurality of aggregation levels.

In further advantageous embodiments, at least one of the search patternscan comprise more candidates from smaller aggregations level than fromlarger aggregation levels and/or at least one of the search patternscomprises more candidates from larger aggregations level than fromsmaller aggregation levels.

Additionally, the invention can relate to a receiving apparatus forreceiving control information within a subframe of a multi-carriercommunication system supporting carrier aggregation, the receivingapparatus comprising: a receiving unit for receiving a subframe from atransmitting node; and a detecting unit for performing a blind detectionfor the control information within a search space by means of a firstsearch pattern, wherein the first search pattern is one of a pluralityof search patterns, each of the plurality of search patterns comprisinga plurality of candidates distributed on any of a plurality ofaggregation levels and wherein the plurality of search patterns furthercomprises a second search pattern whose candidates are non-overlappingthe candidates of the first search pattern on the same aggregationlevels.

Additionally, the invention can relate to a transmitting apparatus fortransmitting control information for at least one receiving node withina subframe of a multi-carrier communication system supporting carrieraggregation, the transmitting apparatus comprising: a mapping unit formapping control information for the receiving node onto a search spaceby means of a first search pattern, wherein the first search pattern isone of a plurality of search patterns, each of the plurality of searchpatterns comprising a plurality of candidates distributed on any of aplurality of aggregation levels, a transmitting unit for transmittingthe subframe to the receiving node, wherein the plurality of searchpatterns further comprises a second search pattern whose candidates arenon-overlapping the candidates of the first search pattern on the sameaggregation levels.

Additionally, the invention can relate to a channel structure forcarrying control information for at least one receiving node within asubframe of a multi-carrier communication system supporting carrieraggregation, wherein the control information is mapped on a search spaceby means of a first search pattern, the first search pattern is one of aplurality of search patterns, each of the plurality of search patternscomprising a plurality of candidates distributed on any of a pluralityof aggregation levels and wherein the plurality of search patternsfurther comprises a second search pattern whose candidates arenon-overlapping the candidates of the first search pattern on the sameaggregation levels.

The above and other objects and features of the present invention willbecome more apparent from the following description and preferredembodiments given in conjunction with the accompanying drawings inwhich:

FIG. 1 is a schematic drawing showing an exemplary downlink componentcarrier of one of two downlink slots of a sub-frame defined for 3GPP LTErelease 8;

FIG. 2 is a schematic drawing illustrating the structure of a non-MBSFNsub-frames and a physical resource block pair thereof defined for 3GPPLTE release 8 and 3GPP LTE-a release 10;

FIG. 3 is a schematic drawing illustrating a structure of MBSFNsub-frames and a physical resource block pair thereof defined for 3GPPLTE Release 8 and 3GPP LTE-A Release 10;

FIG. 4 is a schematic drawing of an exemplary network configurationincluding a donor eNodeB, a relay node, and two user equipments;

FIG. 5 schematically illustrates possible combination of UE scenarios inaccordance with an embodiment of the present invention;

FIG. 6 schematically illustrates search patterns for two UE from thesame UE scenario;

FIGS. 7-10 schematically illustrates search patterns in accordance withembodiments of the present invention;

FIG. 11 schematically illustrates a search pattern configuration inaccordance with an embodiment of the present invention;

FIG. 12 schematically illustrates a further pattern design in accordancewith an embodiment of the present invention;

FIG. 13 schematically illustrates a search pattern configuration inaccordance with an embodiment of the present invention;

FIG. 14 schematically illustrates a search pattern configuration inaccordance with an embodiment of the present invention; and

FIG. 15 schematically illustrates further search patterns in accordancewith an embodiment of the present invention.

Thanks to the search space design of the present invention it ispossible to avoid the complexity of full flexibility, while providingsufficient choices for different scenarios with limited number of blinddecoding trials.

In the following, it is assumed that legacy PDCCH concept is reused,i.e. one ePDCCH is aggregation of {1, 2, 4, 8} eCCEs. It is also assumedthat one PRB pair is divided into four eCCEs.

With reference to FIG. 5, a number of different scenarios can be definedin the following manner. According to UE position, there are mainlythree scenarios:

-   -   1. scenario 5101 comprising cell-center UEs, which can be        configured, for instance, with more lower aggregation level        candidates;    -   2. scenario 5103 comprising cell-middle UEs can be configured        with some higher aggregation level candidates and some lower        aggregation level candidates    -   3. scenario 5102 comprising cell-edge UEs can be configured with        more higher aggregation level candidates; and

At the same time, according to UE feedback, there are mainly threescenarios:

-   -   i. scenario 5201 comprising UE with more accurate feedback, for        instance moving at low speed, preferably using localized        candidates;    -   ii. scenario 5202 comprising UE with less accurate feedback, for        instance moving at high speed, preferably using distributed        candidates; and    -   iii. scenario 5203 comprising UE with roughly accurate feedback,        preferably using both localized and distributed candidates.

Accordingly, in order to provide a targeted search space for allpossible combinations of scenarios 5101-5103 and scenarios 5201-5203,nine possible search patterns have to be defined. However, associatingone search pattern to each possible combination may cause blocking.Moreover, such an approach makes it difficult to pack different DCImessages within the same PRB pair.

For instance, with reference to FIG. 6, it can be seen how UE1 and UE2,both being, for instance, cell-middle UEs with less accurate feedback,would have the same search pattern. Accordingly, this makes it difficultto multiplex search spaces from different UE within the same PRB pair.In fact, in such a situation, only spatial multiplexing is possible by,as indicated in the figure, allocating UE1 to AP8 and UE2 to AP7.However, if there are many such kinds of UEs in the system, blockingamong search space becomes increasingly critical.

This can be improved by providing a plurality of search patterns havinga certain number of candidates for one or more aggregation levels insuch a manner to avoid overlapping of search patterns on the sameaggregation level for at least two patterns.

More specifically, FIG. 7 schematically illustrates two patterns,pattern 0 and pattern 1, in accordance with an embodiment of the presentinvention. In particular, in FIG. 7 the horizontal axis represents theVRB index; the vertical axis represents the AP value while the remainingaxis represents the aggregation level. The two patterns 0 and 1 compriseeach a plurality of candidates arranged on any of aggregation levels 1,2, 4 and/or 8. As can be seen, pattern 0 has candidates on aggregationlevel 1 and aggregation level 2. Similarly, pattern 1 also hascandidates on aggregation level 1 and on aggregation level 2.Additionally, the two patterns are designed so that they arenon-overlapping. In particular, the mapping of the candidates onaggregation level 1 of pattern 0 does not overlap with candidates onaggregation level 1 of pattern 1. Similarly the mapping of thecandidates on aggregation level 2 of pattern 0 does not overlap withcandidates on aggregation level 2 of pattern 1.

Alternatively, or in addition, patterns 0 and 1 are designed such thatthe same aggregation levels and the corresponding number of candidatesare present. Further alternatively, or in addition, the mapping ofcandidates to eCCEs is complementary in both sides for the respectiveaggregation levels, that is, eCCEs for aggregation level 1 in pattern 0are not used for aggregation level 1 in pattern 1, and similarly foraggregation level 2.

By defining pattern 0 and pattern 1 in such a manner, packing, i.e.multiplexing, of different DCI messages in the same PRB is achievedsince the patterns do not overlap. In particular, DL and UL assignmentsto the same UE are possible to be transmitted in the same PRB pair.Moreover, since both pattern 0 and pattern 1 define the same number ofcandidates on the same aggregation levels, they can be applied todifferent UEs in the same scenario, for instance they could be applied,respectively, to UE1 and UE2 of FIG. 6, without overlapping. Thisprovides more flexibility as the number of possible active UEs can beincreased without blocking arising on the channel.

Alternatively, or in addition, FIG. 8 schematically illustrates afurther criterion for the definition of a further search pattern, inaccordance with an embodiment of the present invention.

In particular, pattern 0 of FIG. 8 corresponds to pattern 0 alreadydefined in FIG. 7. Pattern 3, illustrated in FIG. 8 is constructed so asto provide higher aggregation level candidates, compared to pattern 0,while still providing non-overlapping candidates on aggregation level 2with respect to pattern 0. This provides the possibility of employing atthe same time, both pattern 0 and pattern 3.

Moreover, this allows DCI messages from UEs configured with higheraggregation level candidates to be multiplexed with DCI messages fromUEs configured with more lower aggregation level candidates. Even forthe same UE, candidates of aggregation levels 1, 2 and 4 can beconfigured so that US search space does not need to be reconfigured evenif UE scenario changed.

Additionally, this design is advantageous since it allows differentpatterns to have candidates on different aggregation level. Forinstance, cell-center UEs can be associated with patterns having loweraggregation level candidates, such as pattern 0. At the same time,cell-edge UEs can be associated with patterns having higher aggregationlevel candidates, such as pattern 3. In this manner, with a limitednumber of blond decoding trials, different UEs can be configured withdifferent numbers of lower aggregation level candidates and higheraggregation level candidates.

Alternatively, or in addition, FIG. 9 schematically illustrates afurther criterion for the definition of a further search pattern, inaccordance with an embodiment of the present invention.

In particular, FIG. 9 illustrates a pattern 4 in which only candidatesfrom the largest aggregation level are used. Such an approach providesthe advantage that spatial and/or frequency diversity can be obtained,at least for the largest aggregation level, as a fallback mode.Moreover, another benefit is that since aggregation level 8 candidatescan easily block candidates of other aggregation levels, pattern 4 canalways be configured on another antenna port to avoid blocking ofcandidates of other aggregation levels.

Although in the above embodiments only five search patterns have beendefined, the present invention is not limited thereto and the number ofpatterns can be increased, by constructing other patterns in accordancewith the rules given above, or reduced.

FIG. 10 schematically illustrates the combination of five potentialsearch patterns in accordance with an embodiment of the presentinvention.

As can be seen pattern 0 and 1, as well as pattern 2 and 3 offercomplementary candidates. This is turn allows packing of different DCImessages in the same PRB(s). Moreover, pattern 0 and 1 offer candidatesfor lower aggregation level, while Pattern 2 and 3 offer candidatesmainly for higher aggregation levels. This is beneficial since with alimited number of blind decoding trials, different UEs can be configuredwith different number of lower aggregation level candidates and higheraggregation level candidates. Additionally, pattern 4 offers candidatesfor AL 8 so that spatial and/or frequency diversity can be obtained atleast for the largest aggregation level as fallback mode. Moreover, itcan be seen that the patterns are such that candidates are notoverlapping on the same aggregation level.

When employing the search patterns as described above, it is possible todefine a search space by configuring the patterns with the followingparameters:

-   -   pattern ID, such as pattern 0, 1, 2 and/or 3, as defined above;        and/or    -   antenna port, determining which DM-RS port is used to demodulate        the ePDCCH; and/or    -   RB set, determining on which RBs the eCCEs should be detected;        and/or    -   diversity configuration, determining whether e.g. LVRB, DVRB,        SFBC is used for mapping on PRB.

In particular, the antenna port can be used to define the DM-RS port thepattern is mapped to, thereby defining the spatial domain. The advantageof such parameter is that it offers spatial scheduling gain therebyallowing more candidates in the spatial domain and that it offers thepossibility to more candidates to avoid blocking. The RB set can be usedto determine the set of RBs the pattern is mapped to thereby definingthe frequency domain. The advantage of such parameter is that offersfrequency scheduling gain thereby allowing more candidates in thefrequency domain and that it offers the possibility to more candidatesto avoid blocking as well. Finally, the diversity configuration can beused to determine whether, for instance, LVRB, DVRB, SFBC is used formapping on PRB. The advantage of such parameter is that it offersspatial and/or frequency diversity when the channel is not known suchas, for instance, when frequency/spatial selective scheduling is notfeasible.

An exemplary configuration is schematically illustrated in FIG. 11, inaccordance with an embodiment of the present invention.

In particular, the configuration comprises:

-   -   a UE1 being a cell-middle UE with less accurate feedback, as in        the case of FIG. 6, and configured with pattern 3 on AP8, in        distributed mode, and Pattern 4 on AP7 in distributed mode; and    -   a UE2 being a cell-middle UE with less accurate feedback, as in        the case of FIG. 6, and configured with pattern 2 on APB, in        distributed mode, and Pattern 4 on AP7 in distributed mode.

Accordingly, UE1 and UE2 being in similar conditions can usecomplementary patterns so as to achieve the same performances. Thanks tosuch configuration, AL2 and AL4 candidates of UE1 and UE2 search spacescan be multiplexed within one PRB pair since patterns 2 and 3 arecomplementary. Accordingly, this allows packing, in other words,multiplexing, of different DCI messages in the same PRB(s). At the sametime, there is no blocking of AL2 and AL4 candidates from UE1 and UE2.Additionally, pattern 4 contains two AL8 candidates; accordingly thereis no blocking of AL8 candidates from UE1 and UE2. Moreover, since AL8candidates are configured on AP7, there is no blocking between AL8candidates and AL2/AL4 candidates.

Accordingly, the above described configuration based on the abovedescribed patterns provides sufficient flexibility of search spaceconfiguration for different UE scenarios with limited complexitycompared with full flexibility

In particular, in full flexibility the number of candidates is equal to[N _(PRB)·4·12·2]⁴⁰ =560 bitswhere NPRB is the number of PRBs within the whole bandwidth. Forinstance, NPRB is equal to 100 for 20 MHz. 4 is the number of APs,12 isthe number of candidates within one PRB pair, and 2 is the number ofdiversity choices.

On the other hand, with the present invention, the number of candidatesis equal to

${\begin{pmatrix}N_{PRB} \\4\end{pmatrix} \cdot 4 \cdot 12 \cdot 2} = {28\mspace{14mu}{bits}}$per each pattern. If a maximum of 3 or 4 patterns, for instance, isconfigured for one UE, then 84-112 bits are required. Accordingly, thepresent invention uses a very reduced signaling overhead, when comparedwith full flexibility approach.

Moreover, the invention supports fallback mode by obtaining frequencyand/or spatial diversity at least for the largest aggregation level.Additionally, it support frequency ICIC by allowing packing of multipleDCI messages in the same PRB(s). Moreover, it avoids blocking ofcandidates by different DCI messages both from the same or different UE.Finally, it provides a SS framework, which allows operating variousdifferent network policies to schedule and/or configure ePDCCHs, forinstance depending on operating preferences and/or deployment scenarios.

While in the above described embodiment a search space is defined byconfiguring a set of patterns having as parameters the antenna port,and/or the RB set and/or the diversity configuration, the presentinvention is not limited thereto.

Alternatively, or in addition, an applicable set of subframes can beadded to the search space configuration, thereby providing a time domaindiversity as well. In particular

-   -   on high-interference subframes, and/or when common search space        needs to be monitored, a larger number of higher aggregation        level candidates, that is, patterns, can be configured, while    -   on low-interference subframes, a larger number of lower        aggregation level candidates, that is, patterns, can be        configured, so as to save resources.

As an example, the set of subframes can be tied to the subsetdefinitions for CSI reporting. Alternatively, or in addition, the set ofsubframe can be tied to low-power ABS subframes and non-lower-power ABSsubframes.

FIG. 12 illustrates a further pattern design separated by aggregationlevels in accordance with an embodiment of the present invention.

In this embodiment, the patterns are designed according to aggregationlevels. In particular, each pattern contains candidates of oneaggregation level. Moreover, for aggregation level 1, 2 and 4, there aretwo patterns that are complementary to each other. Additionally, thefigure illustrates, to the right of each pattern, the correspondingnumber of candidates, such as Nc=8 for patterns 0 and 1.

This solution provides the benefit of a more flexible combination andconfiguration of the patterns.

FIGS. 13 and 14 schematically illustrate search pattern configurationsin accordance with further embodiments of the present invention. Inparticular, in FIG. 13, a cell-center UE with less feedback isconfigured with pattern 0 and 1 from FIG. 10, with distributedtransmission. In particular, the top part of FIG. 13 illustrates the twopatterns:

-   -   SS1: Pattern 0, AP 7, VRB set 0, DVRB    -   SS2: Pattern 1, AP 8, VRB set 0, DVRB

while the bottom part of FIG. 13 illustrates the resultingconfiguration.

Additionally, in FIG. 14, a cell-center UE with less feedback isconfigured with pattern 2, 3 and 4 from FIG. 10, with distributedtransmission. In particular, the top part of FIG. 14 illustrates thethree patterns:

-   -   SS1: Pattern 2, AP 7, VRB set 0, DVRB    -   SS2: Pattern 3, AP 7, VRB set 0, DVRB    -   SS3: Pattern 4, AP 8, VRB set 0, DVRB

while the bottom part of FIG. 14 illustrates the resultingconfiguration.

Moreover, FIG. 15 schematically illustrates further search patterns inaccordance with an embodiment of the present invention.

In particular, in FIG. 15, all the candidates within one pattern do notoverlap with each other, so that there is no blocking of candidateswithin one pattern. Alternatively, or in addition, pattern 0 and 1, aswell as 0 and 3, have complementary candidates. Further alternatively,or in addition, aggregation level 2 on pattern 3 and aggregation level 1on pattern 0 offer complementary candidates.

The invention claimed is:
 1. A transmitting apparatus comprising:circuitry which, in operation, maps downlink control information to atleast one of a first search space or to a second search space, the firstsearch space including a first set of PDCCH candidates corresponding toa plurality of aggregation levels, the second search space including asecond set of PDCCH candidates corresponding to the plurality ofaggregation levels and an aggregation level higher than any one of theplurality of aggregation levels, wherein for each of the plurality ofaggregation levels a resource for each PDCCH candidate of the first setof PDCCH candidates does not overlap with a resource for any PDCCHcandidate of the second set of PDCCH candidates; and a transmitter whichis coupled to the circuitry and which, in operation, transmits themapped downlink control information.
 2. The transmitting apparatusaccording to claim 1, wherein the first set of PDCCH candidates includedin the first search space are allocated for localized transmission; andthe second set of PDCCH candidates included in the second search spaceare allocated for distributed transmission.
 3. The transmittingapparatus according to claim 1, wherein resources for the first set ofPDCCH candidates do not overlap with resources for the second set ofPDCCH candidates.
 4. The transmitting apparatus according to claim 1,wherein the first set of PDCCH candidates and the second set of PDCCHcandidates are allocated for distributed transmission.
 5. Thetransmitting apparatus according to claim 1, wherein at least one of thefirst search space and the second search space defines a smaller numberof PDCCH candidates as the aggregation level gets higher.
 6. Atransmitting method comprising: mapping downlink control information toat least one of a first search space or to a second search space, thefirst search space including a first set of PDCCH candidatescorresponding to a plurality of aggregation levels, the second searchspace including a second set of PDCCH candidates corresponding to theplurality of aggregation levels and an aggregation level higher than anyone of the plurality of aggregation levels, wherein for each of theplurality of aggregation levels a resource for each PDCCH candidate ofthe first set of PDCCH candidates does not overlap with a resource forany PDCCH candidate of the second set of PDCCH candidates; andtransmitting the mapped downlink control information.
 7. Thetransmitting method according to claim 6, wherein the first set of PDCCHcandidates included in the first search space are allocated forlocalized transmission; and the second set of PDCCH candidates includedin the second search space are allocated for distributed transmission.8. The transmitting method according to claim 6, wherein resources forthe first set of PDCCH candidates do not overlap with resources for thesecond set of PDCCH candidates.
 9. The transmitting method according toclaim 6, wherein the first set of PDCCH candidates and the second set ofPDCCH candidates are allocated for distributed transmission.
 10. Thetransmitting method according to claim 6, wherein at least one of thefirst search space and the second search space defines a smaller numberof PDCCH candidates as the aggregation level gets higher.